The present invention relates to a method and equipment for cleaning and maintaining the rolls of paper or board machines, and especially the rolls of finishing equipment.
Calendars comprise two or more hard and/or soft-surfaced calender rolls, which together form a calender nip or nips through which the paper web to be treated is made to pass. Calender rolls, especially soft-surfaced rolls such as a supercalender's paper rolls or corresponding, and rolls equipped with a so-called soft calender's soft cover, especially a polymer cover, are sensitive to damage. The cause of damage is often impurities, such as local fiber agglomerations, that cause a pressure impact while passing through the nip, which loads the soft cover of the calender roll causing it first to heat (the so-called hot spot phenomenon) and in time causing permanent deformation of the cover and damage. Corresponding deformations and damage can also appear in metallic calender roll surfaces and in the surfaces of belts that pass through the calendering nips.
The damage-induced heating of a calender roll is especially harmful for rolls with polymer covers. Due to the polymer's large heat expansion coefficient and extremely poor heat conductivity, even a small damaged area expands quickly and continues to heat to such high temperatures that it becomes deformed. If a covered roll is made of thermosetting resin, it loses its original properties at the same time as it melts again. The above-described heating reaction can be triggered, for example, by a small piece of paper stuck on the surface of the roll, a fiber agglomeration or a spot released from a paper or board cover which, when it enters a calender nip, causes greater flex than in its immediate surroundings in the cover, which heats the roll cover unevenly.
The prevention of local soiling by, for example, continuous doctoring is not usually feasible financially or with respect to the best quality of the paper or board, because most of the polymer covers in use are not very friction-resistant, so that preventive cleaning may itself wear out the cover more than the actual calendering.
If soiled areas could be detected early enough, it would be possible to use, for example, a cleaning doctor or other device for cleaning the roll surface for short periods without the roll cover being damaged by continuous or often repeated doctoring. Thus, the operating life of soft-surfaced calender rolls could be notably lengthened.
As is already known, thermometers traversing in a crosswise direction with respect to the machine, that monitor the temperature of the cover, are used to monitor the condition of calender rolls, especially soft-surfaced rolls. Problems arise in the said temperature monitoring application and in other corresponding already-known systems, due to the fact that the flexible roll cover whose temperature is being monitored is usually at least somewhat dielectric. Thus, the partially frictional contact between the paper web and the cover produces quite high charges of static electricity on the surface of the cover and the somewhat dry paper web. These potential differences attempt to discharge through the available routes of smallest resistance. The thermograph often has to be installed externally in the support structure, in which case the said charges of static electricity find the thermograph to be the easiest outlet for discharge, thus causing the sensitive electronics of the thermograph to be exposed to quite high voltages and to require protection against them.
Even though monitoring the temperature of the calender roll surface usually makes it possible to detect excessive loading on the cover or a local temperature rise due to local internal non-homogeneity at a sufficiently early stage, it requires installation of quite heavy, expensive and space-taking equipment in the proximity of the roll to be monitored. Especially the lack of space causes great difficulties in connection with multi-roll finishing equipment, in which each device that is not part of the actual paper web finishing process complicates maintenance and repair of the equipment.
Another known method of detecting the soiling of a calender roll surface is a measuring method based on roll vibration, which is presented in the applicant's patent application FI 974255. In this method, one or more vibration sensors that indicate and localize the vibration point caused by soiling are inserted in the calender roll structure. Soiling can also be detected by a fault detection system that monitors the state of the paper web. This system detects holes and coating color streaks in the paper web. Other known systems that detect roll soiling include a machine condition monitoring system, a quality measurement system, web moisture measurement and the above-explained web temperature measurement.
When the soiling of a roll has been detected, an attempt is made to remove the soiling as quickly as possible to prevent damage to the surface of the roll and to minimize the amount of rejected product. Most commonly, the dirt is removed by using a conventional blade doctor, in which the doctoring blade extends for the whole length of the calender roll and usually continuously removes, i.e. doctors, the soiling and impurities. Other known doctoring methods include a washing doctor that directs a liquid jet on the soiled part, and a flow doctor that directs an air jet on the part to be cleaned. A set of calender rolls can also be supplemented with a cleaning nip, which removes the worst coat weight deviations that come with the paper web before they are transferred to the set of calender rolls.
The problem of the conventional blade doctor is that the doctoring function is applied to the roll surface continuously, which leads to unnecessary wearing of the roll, thus shortening the roll's operating life.